The present invention relates generally to optics, and more particularly, to a diffractive optical element that provides favorable multi-mode fiber launch and reflection management.
A vertical cavity surface emitting laser (VCSEL) emits light in a beam vertically from it surface. Light emitted from a VCSEL is typically focused by a transfer lens into an optical fiber and used for the transmission of data. Transmission technology such as Gigabit Ethernet technology utilizes VCSELs and multimode fiber optic cabling.
The ever-increasing data rate across multimode fiber optic systems requires more sophisticated coupling optics for the transmitter module to satisfy the required bit-error rate.
There are two important considerations in the design of a transfer lens: 1) reflection management, and 2) creation of a favorable launch condition. The first design consideration of reflection management seeks to minimize the amount of light that is reflected back from the surface of the optical fiber (referred to as xe2x80x9cback reflectionsxe2x80x9d or feedback) and directed to the light source (e.g., the laser). When reflections are not managed properly, the back reflections can cause stability problems for the laser source. Specifically, if these back reflections are not controlled or reduced, the laser can become de-stabilized and may operate with a noisy output signal. For example, when too much power is coupled back into the laser from the reflection from the end of the optical fiber, instabilities occur in the laser, and the output power oscillates up and down, thereby causing extra and damaging amounts of jitter as the received signal pulses. In other words, instability in the laser causes erroneous data signals.
Furthermore, the increased noise in the laser that is induced by the coupling lens can lead to a power penalty in the optical budget of the data link as high as 2.5 dB. It is evident that the increased power penalty due to the back reflections represents a significant fraction of the total link power budget which, for a 2.5 Gbit/sect data rate, is on the order of about 8 dB. This adverse effect of back reflections or feedback becomes more pronounced and significant for higher data rate systems. For example, the power budget for a 10 Gbit/sec link becomes even more taxed than the 2.5 Gbit/sec link.
Second, it is important that the transfer lens design provide a favorable launch condition at the fiber interface in order to maximize the bandwidth-distance product of the system. For example, for a standard 50 micron graded-index fiber, a 2.5 Gbit/sec link requires a bandwidth-distance product of 500 MHz*km. Similarly, for a 10 Gbit/sec link, the fiber needs to support a product of 2.2 Ghz*km.
A favorable launch condition should increase bandwidth of the system and is robust to lateral offsets (i.e., misalignment between the laser and the fiber). One approach to improve favorable launch condition is to avoid launching the light along the very center of the fiber. A reason for avoiding the center of the fiber is that many fibers have defects along the center of the fiber due to manufacturing limitations. Furthermore, tolerance for lateral offsets is desirable to compensate for any misalignment between the laser and the fiber. Otherwise, misalignment in the system (e.g., misalignment between the optical fiber and transfer lens or the misalignment between the transfer lens and the laser) may cause the light from the laser to miss the optical fiber.
Unfortunately, the prior art transfer lens designs have shortcomings in either addressing back reflections or providing favorable launch conditions. These shortcomings and disadvantages stem primarily from constraints and difficulties in lens fabrication.
Diffractive Vortex Lens for Mode-Matching Graded Index Fiber
There have been some attempts to use diffractive elements as coupling optics to launch light into graded index fiber. One such study is reported by E. G. Johnson, J. Stack, C. Koehler, and T. Suleski in the Diffractive Optics and Micro-Optics, Optical Society of America (OSA) Technical Digest, pp. 205-207, Washington, DC, 2000, in an article entitled, xe2x80x9cDiffractive Vortex Lens for Mode-Matching Graded Index Fiber.xe2x80x9d This publication describes an approach that utilizes a diffractive element to match the phase of the launched light into specific modes of the graded index fiber.
While these prior art approaches provide tolerable results for ideal point light sources (i.e., light that has a simple distribution and is perfectly coherent), these approaches do not adequately address applications that employ light sources with more complex light distributions (e.g., a multi-mode laser). In these specific real-world applications, the prior transfer lens suffer from more destabilizing feedback due to poor management of back reflections, unfavorable launch conditions stemming from larger amounts of on-axis energy, or both.
Based on the foregoing, there remains a need for a transfer lens that simultaneously reduces back reflection and provides favorable launch conditions and that overcomes the disadvantages set forth previously.
According to one embodiment of the present invention, a transfer lens for transferring light emitted by a light source (e.g., a laser) into a light conducting medium (e.g., an optical fiber) is provided. The transfer lens includes a diffractive surface for receiving and collimating the light originating from the light source. The diffractive surface is defined by a surface function that includes a first phase function having angular symmetry and a second phase function having radial symmetry. The second phase function includes a cusp region with a discontinuous slope therein. The transfer lens provides reflection management so that light reflected from the end of the optical fiber is not focused at a location at which light is emitted by the light source. Furthermore, the transfer lens also provides favorable launch conditions so that light launched into the optical fiber avoids index anomalies on the axis of the optical fiber and at the core-cladding interface.
A further advantage of the transfer lens design of the present invention is that the diffractive surface of the transfer lens provides reflection management and favorable launch conditions, which is particularly advantageous for multi-mode fiber optic systems.
In accordance with one embodiment of the present invention, a light transmission system that includes a laser, an optical fiber, and a transfer lens is provided. The transfer lens transfers light emitted by the laser into the optical fiber. The transfer lens includes a diffractive surface for receiving and collimating the light originating form the laser. The diffractive surface is defined by surface function that includes a first phase function having angular symmetry and a second phase function having radial symmetry. The second phase function includes a cusp region with a discontinuous slope therein. The transfer lens provides reflection management so that light reflected from the end of the optical fiber is not focused at a location at which light is emitted by the laser and also favorable launch conditions so that light launched into the optical fiber avoids index anomalies along the axis of the optical fiber.